By Kim Eun-jung SEOUL, Dec. 7 (Yonhap) -- North Korea is filling up a fuel tank at a missile launch site as it prepares to inject fuel into a long-range rocket that Pyongyang says would blast off as early as Monday, a senior military source in Seoul said Friday.

According to the source, the South Korean government has spotted increased activities near the fuel storage at the Dongchang-ri launch site in the North's northwest, where a three-stage rocket has been installed since earlier this week.

"As soon as (the North) completes injecting fuel into the storage, it is expected to supply the rocket with fuel," the source said on the condition of anonymity. "Fuel injection could begin Saturday."

A satellite image shows North Korea is preparing fuel injection to fire off a long-range rocket in the launch site in the nation's northwestern area. (Yonhap)

An image taken by Korea's multipurpose Arirang-3 satellite Thursday afternoon shows an air compressor and a tanker standing near the 50-meter-high launch site, which is located about 80 meters away from the fuel storage tank.

There were cars, trucks and special vehicles near the control center and the accommodation area.

The image also showed two trailers, which delivered the first and second stages of the rocket, and a tow truck were parked near the assembly line.

The source said the latest image indicates that the North is examining the rocket's body and the observation post is inspecting the communications network, noting it is hard to detect the actual fuel injection because the fuel pipeline is installed underground.

Experts say the North is expected to set a launch date depending on weather conditions as soon as the rocket is ready to go, considering severe weather conditions in the northern region in winter.

While Pyongyang announced its plan to fire off the rocket sometime between Dec. 10-22, weather agencies forecast the first day will be cloudy and next Wednesday will be clear.

Japanese media earlier reported that leader Kim Jong-un may fire off the rocket on the morning of Dec. 17, the first anniversary of the death of his father Kim Jong-il, in a tribute to the late leader.

The anniversary comes just two days before South Koreans go to polls to elect a new president.

It would be the second rocket launch attempt under the untested young leader, believed to be in his late 20s, who took power after his father died nearly a year ago. The April test failed shortly after liftoff.

Although Pyongyang says the launch aims to put a "working satellite" into space, Seoul, Washington and many other countries view it as a test of a long-range ballistic missile that would violate U.N. resolutions.

Lim Sung-nam, Seoul's top nuclear envoy, said he discussed the issue with U.S. officials during his four-day visit to Washington earlier this week.

"(We) agreed that if North Korea launches the missile, it would not only be a violation of U.N. Security Council resolutions but a frontal challenge against the international community," Lim told Yonhap News Agency by phone after returning to Seoul earlier in the day. "We agreed to first maximize our diplomatic efforts to stop the launch."

He added, however, that Seoul and Washington will start preparing for U.N.-led action as North Korea could still go ahead with the launch.

The South Korean military has stepped up intelligence gathering to prepare for the rocket launch, deploying warships with radar to the Yellow Sea to track the rocket during its launch and flight.

The Chosun Sinbo, a Tokyo-based newspaper seen as a mouthpiece of the Pyongyang regime, reported that the North is planning to build a larger rocket if the planned launch succeeds.

"(North Korea) will begin to develop an even larger rocket than the Unha-3, which will carry large loads," the paper said.